The present invention relates in general to subsea storage and cleansing of drill cuttings from holes drilled in the sea bed, especially in connection with hydrocarbon exploration. More particularly the invention relates to a storage structure designed to be positioned on the sea bottom for storage and cleansing of drill cuttings continuously delivered from nearby drilling operations.
Furthermore, the invention relates to a method for cleansing and separation of drill cuttings from oils, fluids and other contaminating constituents.
In connection with drilling operations in the sea bed drilling fluids and drilling slurries are pumped down during the drilling for different purposes, such as cooling, lubrication of the drill bits, prevention of corrosion, and also in order to create a pressure barrier in order to sustain those substantial pressures which may occur in the drill well when reaching ground formations with high pressures. Two main types of drilling fluids and drilling slurries are used, namely water-based and oil-based. Oil-based drilling fluids are primarily used to provide a lubrication of the drill string in the well and also to prevent corrosion of the drilling equipment. Oil-based drilling fluids are further used for drilling at high temperatures, in order to improve well stability and reduce scaling, and in order to control possible risks of water penetration. While drilling fluids and slurries are primarily used in order to create a pressure barrier in connection with high pressures in the well, they also serve as a supporting vehicle for the well cuttings which are generated during the drilling. Drill cuttings, contaminating oils, drilling fluids and the like are continuously transported up to the surface, i.e. to the drilling platform or drilling vessel where the drill cuttings and the drilling fluids to a certain degree are filtered for recovery of the drilling slurries. The drill cuttings are thereafter usually dumped in the sea and come to rest at the sea bottom.
The drilling cuttings normally contain both oil and various chemicals which usually are located as a film encompassing the drill cutting particles. If the cuttings are disposed of by dumping into the sea, oil and chemicals detach from the cutting particles and then normally will float up and form oily films on the sea surface, thereby contaminating the sea water.
One is here speaking of considerable quantities and volumes. Asan example, it can be mentioned that in the North Sea during the first six months of 1989 it was planned to drill altogether 125 oil and gas wells, and the drill cuttings from a single well may constitute approximately 500 metric tons. Knowing that oil contaminated drill cuttings may contain up to 50 volume % oil-based fluid one will understand that the risk of substantial environmental contaminations is rather great.
In this technical field great efforts have therefore been made to provide steadily improved methods for cleaning of the drill cuttings. For separation and cleansing of the drill cuttings are utilized both burning and/or chemical methods. It is obvious that such methods will be expensive, and the results have hitherto not been satisfactory.
An alternative to separation and cleansing is to accumulate the drill cuttings for transporting the same ashore with barges or the like, possibly for further cleansing and storage ashore. Such procedures obviously are rather expensive and new problems will arise in connection with storage ashore.